As others have said, do try and make time for DC1. I also found that it was a real divide of me always with and breastfeeding the baby and DH always with the oldest one - I missed them and they felt ousted out of my life. The baby needs to be loved and fed and cuddled but they don't have this massive shellshock to their world that older children do.
Even though you'll be knackered, try and plan in individual time each day with DC1 - even if it's just five minutes to read a quick story or have a laugh about something you both enjoy and used to do together.
I also tried to get our youngest into a gentle routine as soon as possible. I would tuck her up on her bouncy chair or in the pram next to the washing machine and tumble dryer and the drone would help her to nod off. This meant that I knew as soon as her naps were fairly regular that I would have time each day to help older DC and I could say "When DD3 has her sleep then I will cut out that for you, read you that story etc".
I also found getting out as much as possible helped - I couldn't sleep during the day with older ones awake and the housework wasn't going to get done anyway so getting out helped us all enormously.
Playgroups were great as DD3 would sleep in her pram or car seat and I could spend time with older DDs. In fact I used to go to one every week where I knew no one just because this meant I could concentrate solely on my older girls - rather than them feeling ousted again for me to talk to the other mums!
Try and think ahead too - if I needed to say chop apples for snacks or do something for lunch where I couldn't hold a baby at the sam etime - I would start preparing lunch at 8am if that was the time that our baby slept! Then at least it was in the fridge ready!
I also bought lots of ready soups and easy to eat with one hand things so that lunch and tea weren't too complicated!
Good luck!